Natural killer (NK) cells and killer (K) cells mediating antibody-dependent cellular cytotoxicity have been shown to be large granular lymphocytes (LGLs). The mechanism of cytotoxicity by LGLs can be divided into two clearly distinguishable stages. The first stage is recognition and binding of the LGLs to target cells, while the second includes post-binding events that lead to target cell lysis. Studies are proceeding to define the receptors and structures involved in NK recognition. A monoclonal antibody (MAb) was developed against NK target antigens on K562 cells and that antibody blocks LGL binding and lysis. We also developed an anti-idiotypic antibody (anti-ID) against this MAb anticipating that it might recognize the NK receptor and aid in its identification. This anti-ID antibody is reactive with an effector cell protein and blocks LGLs binding and target cell lysis. LGLs pretreated for 18 hr with the F(ab')2 anti-ID antibody significantly enhanced lysis of target cells and production of interferon-gamma by LGL. It will be important to perform studies to characterize the expression of the putative receptor molecule on activated cells that have been reported to have NK activity. Further biochemical characterization of the NK receptor and its cloning are in progress. The role of soluble factors such as natural killer cytotoxic factor (NKCF) in this cell-mediated lysis has been examined to gain an understanding of the mechanism of lysis. The activity of NKCF has been divided into three distinct stages: a) production; b) binding to targets; and c) target cell lysis. The availability of procedures to independently measure these stages has enabled us to test a variety of agents that inhibit NK cell-mediated killing and to determine the factor's site of action. NKCF is produced by LGLs and its general specificity pattern is similar to that of intact NK cells. NKCF has been demonstrated to be distinct from recombinant lymphotoxin, tumor necrosis factor, and leukoregulin. We are using rat LGL cell lines as a source of NKCF and are pursuing studies leading to its molecular cloning with the aid of the specific MAb to NKCF.